1. Market makers will of course quote their own prices and keep 3 eyes on what their competitors are quoting. This means everyone pretty much quotes the same MID PRICE but you will get slightly different bid-offers.
For example, right now IB is bidding at 1.3137.3 and LMAX (bid) 1.3136.9. The mid price is right around 1.3137.6.
Try to keep it simple, if IB are quoting something, that is the current market and as a trader it doesn't matter too much how/why they're quoting that price, the important thing is there's a tight price being offered - the trader's job is to try and guess if that number is likely to rise or fall.
2. I'm not an expert on what goes on behind the scenes but there's hundreds of millions of USD in profit to be made for the market makers over a year (maybe even billions). So yes, they'll employ all sorts of powerful computers, algos, machine learning, possibly AI, to make the best prices possible. And with heavy competition the bid-offers can only get tighter which is bad for them but great for us traders (cheaper trading). Again, it's not so important to understand how/why they quote, the important thing is the price is tight and we can trade on it.
3. Arbs are just arbing on speed I think between the different liquidity providers. No different to all the different US stock exchanges, buy on one exchange, sell on another for a tiny profit (perhaps 1/10th of a cent) but doing the trade many times a day. The reason this sort of trading is not open for the average trader is it takes a considerable investment. As for dark pools, again I'm not sure if there are or aren't any in FX. All I'm sure about is that IB quotes a great price.
4. There's nothing special about Cable apart from it's where the action is. 10+ years ago EUR-USD was better because it was moving 100-200+ ticks a day with 4-5 times the liquidity of Cable. But the Euro over the last 5 years has been pretty much dead for intraday trading. Perhap in a few years the Euro will get great again and many Cable traders will move. Cable and the Euro as well as Soyabeans are just numbers that go up and down. Not a lot of difference to me and many other traders. Try to pick one that moves and offers good action.
If people want to short term trade Cable I'd say -
1. The 1m chart is all you need. Yes, it's fast but nothing that cannot be handled.
2. Taking profits is hard, most mess it up over a series of trades, even the great traders. Learn to accept this, it's normal.
3. CONCENTRATION is critical. Lose that and the results won't be good, not so much with losses but failing to spot good trades. Even the 1m can seem boring much of the time which can lead to losing concentration. Work hard to keep concentrating...